In these days, much attention is focused on a power line communication technique which communicates various information including an image signal, a sound signal, and a control signal by superimposing the above information over AC power, using an existing commercial AC power line for supplying the AC power to home electric appliances. By introducing the power line communication technique to an AV device, a user has only to connect a power cable to a power receptacle in order to transfer the image signal and the sound signal between devices and there is no need to wire a new cable, which improves user's convenience.
With an increase of unstable situation such as robbery and vicious crimes, there arise a lot of occasions when a surveillance camera with a network function is set in order to deter crimes and record the information at a time of committing a crime. A surveillance camera system is proposed in which the power line communication technique is introduced to a network unit for transmitting and receiving the information bidirectionally between the surveillance camera and the control recording device which controls imaging operations such as pan, tilt, and zoom and records the monitored image photographed.
Generally, the surveillance camera system using the power line communication technique is used by being connected to a commercial AC power line at home. It is supposed that in addition to the surveillance camera system, various home electric appliances are connected to the commercial AC power line. In the AC power lines branched for every breaker by a distribution board, when the total sum of power current consumption of the devices connected to the same AC power line as the surveillance camera system exceeds a rated current, the corresponding breaker of the distribution board is turned off, hence to stop the AC power supply to the surveillance camera system, and the monitored images are not recorded during the time.
In order to solve the problem, a surveillance camera system using the conventional power line communication technique has been proposed: when the AC power supply is stopped as described above, the camera operates by switching the power from AC to a built-in DC power source, photographs static images intermittently at every predetermined time, and temporarily stores the static images in a built-in recording unit as the monitored image. After the AC power supply is restored, it returns to the ordinary operations and transmits the monitored images stored to the control recording device.
Hereinafter, the conventional surveillance camera system will be described.
As the conventional surveillance camera system, a system described in Patent Document 1 is well known. FIG. 3 is a view for explaining a structure of the conventional surveillance camera system. In FIG. 3, camera unit 120 includes camera processing unit 109 which obtains the information of image and sound at a monitored place and performs desired processing to generate a camera signal, first power line communication processing unit 112 which performs the processing necessary for the power line communication on the camera signal to generate a first power line communication information signal, first power source unit 113 which converts the supplied AC power into first DC power, selects either the first DC power or second DC power supplied from first built-in DC power source unit 132, and supplies the power to camera processing unit 109 and first power line communication processing unit 112, first AC/signal superimposition separating unit 115 which transmits the first power line communication signal obtained by superimposing the first power line communication information signal on the AC power to control recording unit 220 through the AC power line and separates the AC power to supply the power to first power source unit 113 when it is connected to the AC power line and the AC power is supplied to the AC power line, and AC power source cable 116.
Camera processing unit 109 includes imaging unit 101 which obtains the image of the monitored place, microphone 102 (hereinafter, abbreviated as “mic”) which obtains the sound of the monitored place, signal processing unit 103 which performs desired processing on the information such as image and sound to generate a camera signal, recording unit 104 which records the camera signal temporarily, and signal selecting unit 105 which selects either the camera signal supplied from signal processing unit 103 or the camera signal supplied from recording unit 104 and supplies the selected signal to first power line communication processing unit 112. First power source unit 113 includes AC/DC converting unit 131 which converts the supplied AC power into the first DC power, first built-in DC power source unit 132 which supplies the second DC power, and power source selecting unit 133 which selects either the first DC power or the second DC power and supplies the power to camera processing unit 109 and first power line communication processing unit 112.
In FIG. 3, control recording unit 220 includes second AC/signal superimposition separating unit 223 which separates the first power line communication information signal and the AC power from the first power line communication signal received from camera unit 120 through the AC power line and supplies them when it is connected to the AC power line and the AC power is supplied to the AC power line, second power line communication processing unit 219 which receives the first power line communication information signal supplied from second AC/signal superimposition separating unit 223 and obtains the camera signal, camera control recording processing unit 217 which receives the camera signal supplied from second power line communication processing unit 219 and records the information such as the image and the sound at the monitored place, second power source unit 214 which converts the AC power supplied from second AC/signal superimposition separating unit 223 into third DC power and supplies the power to camera control recording processing unit 217 and second power line communication processing unit 219, and AC power source cable 116.
Camera control recording processing unit 217 includes camera control unit 271 which generates a control signal for controlling camera unit 120 and camera signal recording unit 272 which records the camera signal supplied from second power line communication processing unit 219. Second power source unit 214 has AC/DC converting unit 211 which converts the supplied AC power into the third DC power.
Camera unit 120 and control recording unit 220 forming the conventional surveillance camera system are connected to the AC power line which is connected to the same breaker in the distribution board through AC power source cable 116. When the AC power is supplied through the AC power line, various kinds of information communication may be simultaneously realized. They don't have to be always connected to the AC power line which is connected to the same breaker on the distribution board but camera unit 120 may be connected to control recording unit 220 through an AC power line of another distribution board. This is true in an embodiment of the invention.
FIG. 4 is a view for explaining a state of superimposing the power line communication information signal over the AC power in the power line communication performed by the conventional surveillance camera system. In FIG. 4, the upper left view shows the AC power of 100V at frequency 50 Hz or 60 Hz supplied to the commercial power line and the lower left view shows the power line communication information signal generated from an image signal, a sound signal, and a control signal which a user wants to transfer through the AC power line. Generally, in the power line communication, the power line communication information signal is superimposed on the AC power, hence to generate a power line communication signal as shown by the right view of FIG. 4, thereby transmitting the information signal through the AC power line. This way of superimposing the power line communication information signal on the AC power is true to the embodiment of the invention.
Owing to the above structure, in the conventional surveillance camera system, while the AC power is supplied, the first DC power converted from the supplied AC power by AC/DC converting unit 131 is supplied to camera processing unit 109 and first power line communication processing unit 112, in camera unit 120. In first power line communication processing unit 112, the processing necessary for the power line communication such as protocol processing and power line communication protocol processing at each layer defined by OSI (Open Systems Interconnection) reference model is performed on the camera signal generated by performing the desired processing on the information such as moving image and sound obtained by imaging unit 101 and mic 102, hence to generate a first power line communication information signal as shown in the lower left view of FIG. 4. First AC/signal superimposition separating unit 115 transmits the first power line communication signal as shown by the right view of FIG. 4 obtained by superimposing the first power line communication information signal on the AC power to control recording unit 220 through the AC power line. In control recording unit 220, second AC/signal superimposition separating unit 223 separates the first power line communication information signal from the first power line communication signal supplied from camera unit 120. In second power line communication processing unit 219, the camera signal obtained from the first power line communication information signal is recorded in camera signal recording unit 272.
While the AC power supply is stopped, in camera unit 120, the second DC power supplied from first built-in DC power source unit 132 is supplied to camera processing unit 109 and the camera signal is stored in recording unit 104 temporarily, which is generated by performing the desired processing on the information of the static images obtained intermittently at every predetermined time by imaging unit 101. After the AC power supply is restored, first power line communication processing unit 112 performs the processing necessary for the same power line communication as the above such as the various kinds of protocol processing on the camera signal generated from the static images stored in recording unit 104, hence to generate the first power line communication information signal as shown in the lower left view of FIG. 4. First AC/signal superimposition separating unit 115 transmits the first power line communication signal as shown in the right view of FIG. 4 obtained by superimposing the first power line communication information signal on the AC power to control recording unit 220 through the AC power line.
In control recording unit 220, second AC/signal superimposition separating unit 223 separates the first power line communication information signal from the first power line communication signal received from the camera unit 120. Second power line communication processing unit 219 records the camera signal obtained from the first power line communication information signal in camera signal recording unit 272.
In the above conventional surveillance camera system, while the AC power supply is stopped, since there is a restriction in the recording capacity of recording unit 104 and consumption of the DC power has to be restrained in first built-in DC power source unit 132, the static images are intermittently obtained at every predetermined time and stored in recording unit 104. Compared with the period when the AC power is supplied, the amount of the information is reduced and there is a possibility of losing important information of the monitored place when the AC power supply is stopped. During the period, the communication with control recording unit 220 is not performed at all, and control recording unit 220 can't remote-control the operation of camera unit 120 such as pan, tilt, and zoom.    Patent Document 1: Japanese Patent Unexamined Publication No. 2004-120187